Sensory systems in mammals consist of multiple pathways from the peripheral receptors to neocortex. Of these, only one is organized for transmitting the precise physical characteristics of sensory stimuli. The proposed experiments are designed to show the organization and function of parallel somatic sensory pathways. Preliminary results indicate that, of the multiple somatic pathways carrying information from the whiskers to the barrel field cortex of rat, only the 'lemniscal' pathway through the medial subdivision of the thalamic ventral posterior nucleus (VPM) is capable of maintaining the sensory responsiveness of neurons in the barrel field. The general thesis is that the 'paralemniscal' pathway through the thalamic posterior nucleus (PO), terminating in septa surrounding cortical barrels, regulates the excitability of septal neurons; by modulating the horizontal distance in cortex across which activity spreads, PO controls the degree of interaction and association between inputs from VPM terminating in separate barrels. To test this hypothesis, we will identify with anatomical transport methods the locations in cortex at which lemniscal and paralemniscal inputs converge. Then we will measure the effect of stimulation of PO on the magnitude and horizontal spread of cortical activity evoked by movement of a single whisker. The PO stimulation will be delivered either synchronously with whisker movement or out of phase with whisker movement. We will then measure the influence of PO stimulation on the plasticity of cortical receptive fields induced by temporally paired movement of whiskers. Finally, we will investigate the role of glutamate receptors in barrel field receptive field plasticity.